Shahed Reza

Self-calibration method for radial GRAPPA/k-t GRAPPA

Generalized autocalibrating partially parallel acquisitions (GRAPPA), an important parallel imaging technique, can be easily applied to radial k‐space data by segmenting the k‐space. The previously reported radial GRAPPA method requires extra calibration data to determine the relative shift operators. In this work it is shown that pseudo‐full k‐space data can be generated from the partially acquired radial data by filtering in image space followed by inverse gridding. The relative shift operators can then be approximated from the pseudo‐full k‐space data. The self‐calibration method using pseudo‐full k‐space data can be applied in both k and k‐t space. This technique avoids the prescans and hence improves the applicability of radial GRAPPA to image static tissue, and makes k‐t GRAPPA applicable to radial trajectory. Experiments show that radial GRAPPA calibrated with pseudo‐full calibration data generates results similar to radial GRAPPA calibrated with the true full k‐space data for that image. If motion occurs during acquisition, self‐calibrated radial GRAPPA protects structural information better than externally calibrated GRAPPA. However, radial GRAPPA calibrated with pseudo‐full calibration data suffers from residual streaking artifacts when the reduction factor is high. Radial k‐t GRAPPA calibrated with pseudo‐full calibration data generates reduced errors compared to the sliding‐window method and temporal GRAPPA (TGRAPPA). Magn Reson Med 57:1075–1085, 2007.

1/f noise in metallic and semiconducting carbon nanotubes

The charge transport and noise properties of three terminal, gated devices containing multiple single-wall metallic and semiconducting carbon nanotubes were measured at room temperature. Applying a high voltage pulsed bias at the drain terminal the metallic tubes were ablated sequentially, enabling the separation of measured conductance and 1∕f noise into metallic and semiconducting nanotube contributions. The relative low frequency excess noise of the metallic tubes was observed to be two orders of magnitude lower than that of the semiconductor tubes.

Thermally activated low frequency noise in carbon nanotubes

The low frequency noise of single-walled carbon nanotubes is studied over the 77–300K temperature range. Lorentzian shaped spectra along with 1∕f noise spectra have been observed. From the Lorentzian noise components, a range of thermal activation energies from 0.08to0.51eV for the associated fluctuation mechanisms is obtained. From the 1∕f noise spectra, a distribution of activation energies of fluctuation processes ranging from 0.2to0.7eV is derived. These findings indicate that the observed noise spectra are caused by number fluctuations.

Image guided noise tomography for increased specificity of magnetic resonance imaging

A method for increasing the specificity of an MR image using noise correlation measurements is presented. From an MR image different regions within the body are identified based on contrast. Noise signals measured at the ports of the experimental setup are functions of the conductivity at each region and the sensitivity map of the field probes. For a simulated sensitivity map, the ratio of conductivities of two regions of a phantom containing saline and distilled water was determined from the measured noise correlation at the ports.

TIME REQUIRED FOR A SPECIFIED LEVEL OF ACCURACY IN NOISE CORRELATION MEASUREMENTS

Correlation measurement is an effective way of measuring noise components with a magnitude smaller than the system noise. The accuracy of such measurement depends on the system bandwidth, the magnitude of the noise to be measured compared to the system noise and the measurement time. The accuracy can be improved by increasing the measurement time. A method of estimating the measurement time required for a desired level of accuracy for a given system noise and bandwidth is presented.

Phantom calibration method for improved temporal characterization of hemodynamic response in event-related fMRI.

In event-related functional MRI, there exist limits on the time length of the experiments on human subjects and the imaging speed. Due to these limitations, data truncation and undersampling have to be used in functional MRI signal acquisition. The effect of these factors on the hemodynamic deconvolution is investigated experimentally and a phantom calibration method to improve the hemodynamic response is developed. It is observed that the high frequency components generated due to data truncation can fold back into low frequencies when the sampling rate is not sufficiently high. This aliasing can introduce significant noise in hemodynamic deconvolution and can reduce the accuracy of the temporal characterization of hemodynamic response. A SMARTPHANTOM BOLD simulator is used to calibrate the aliasing effect in an event-related functional MRI experiment. With the calibration, an anti-aliasing method is used to suppress the aliasing and this resulted in an improved temporal characterization of hemodynamic response in event-related fMRI.

IMAGE GUIDED NOISE TOMOGRAPHY FOR INCREASED SPECIFICITY OF MAGNETIC RESONANCE IMAGING

A method for increasing the specificity of an MR image using noise correlation measurements is presented. From an MR image different regions within the body are identified based on contrast. Noise signals measured at the ports of the experimental setup are functions of the conductivity at each region and the sensitivity map of the field probes. For a simulated sensitivity map, the ratio of conductivities of two regions of a phantom containing saline and distilled water was determined from the measured noise correlation at the ports.

Low Frequency Excess Noise In Carbon Nanotube Devices

Low frequency noise and resistance data measured on metallic and hole transport semiconducting single wall carbon nanotubes are presented. The data indicate that charge transport is limited by tunneling through the Schottky barrier extended source contacts. Carrier trapping in this region is proposed as the main source of the observed low frequency noise.

Assessing the 1/f noise contributions of accidental defects in advanced semiconductor devices

The effects on the overall device noise of a small number of defects in device sections with a strong transfer impedance coupling to the device terminals is discussed using advanced bulk and silicon-on-insulator n channel MOSFETs and silicon nanowires as examples. From the measured noise and current-voltage data, the precise physical location of the noise centers is determined. Potential noise reduction methods are discussed.

Excess Noise In Carbon Nanotubes

The excess noise of single‐walled carbon nanotubes is studied over the 77K to 300K temperature range. Lorentzian shaped spectra along with 1/f noise spectra are observed. From the Lorentzian noise components activation energies of 0.21 and 0.46eV for the associated fluctuation mechanisms are obtained.